Numerical Simulations Of Coupled Evolutions Between Twins And Grains In Nanotwin Polycrystal

Nano-twinned polycrystal materials exhibit excellent properties of high strength and good ductility due to many nanoscale coherent/incoherent twin boundaries,which show size effect relying on twin length and grain size.Recently,someone finds that the formation of nanotwins is an effective way to assist grain coarsening and growing in gold nanometer films under cyclic loading based on atomic-scale observations and analyses.Exploring and understanding the mechanism of the phenomenon is of great importance for designing nano structured metal with both high strength and good ductility.In this paper,by developing a unified phase field model taking into account grain growth and twin formation concurrently,the detailed effects of different conditions on the evolution of twin formation and grain growth have been studied in bicrystal and polycrystal.In order to observe evolution behavior from atomic scale,we undertake a limited attempt to simulate using phase-field-crystal method for observing evolution behavior.The main content is as follows:Based on the phase field model of grain growth and twin formation,a continuum phase field model has been developed to simulate the relationship between grain growth and twin formation.The connection between eigenstrain and both grain long-range order(lro)parameters and twin lro is built up,and then the total free energy of system is expressed as the function of grain lro and twin lro,finally,evolutions of the grain growth and twin formation are governed by phenomenological time-dependent Ginzburg-Landau kinetic equations.Actually,the physical essence of the model established is the energy competitions between the elastic energy and the interfacial energy based on thermomechanical analysis.We measure the angle of the twin growing strongly anisotropic that is the same as the direction of the twin invariant plane,which confirm the validity and feasibility of the phase field model established in chapter two.In bicrystal,the effects of the applied stress and misorientation of bicrystal on the relationship between grain growth and twin formation are studied.We obtain three different micro structure evolution patterns of grain growth and twin formation under fixed applied stress by changing misorientation of bicrystal.Furthermore,we obtain the phase diagram of three microstructure evolution patterns by a lot of simulations changing the applied stress and misorientation of bicrystal.Expanding the model of bicrystal to polycrystal,generating a structure of polycrystal randomly,we have studied the relationship between the twin formation and grain growth.Comparing the simulation result of the traditional grain growth model and the model established in chapter two,we obtain that twin formation is favor of grain growth.Furthermore,comparing the effects of different applied stress on grain growth using the model established in chapter two,we obtain that there is a critical value,only when the applied stress is larger than it,twins can form and promote grain growth.And the larger the applied stress is,the higher rate of grain growth is,thus will be more significant for twin assisting grain growth.To consider the dynamic factors,we have simulated the situations where the migration rate of grain growth is comparable to twin formation and where the migration rate of grain growth is much smaller than twin formation and where the migration rate of grain growth is much larger than twin formation.We obtain that only when the rate of grain growth is comparable to twin formation,twin formation can assist the grain growth effectively.And we plot the micro structure evolution process between twins and grains in polycrystal structure.Similarly to the model of bicrystal,there are still three different microstructure evolution patterns between grain growth and twin formation.By the phase-field-crystal method,we obtain the atomic arrangement structure of twins.We study the coupled evolution between the twin boundary and the grain boundary of different misorientation under uniaxial compression,and find that the grain boundary migrate partly,but the degree of grain boundary migration is so small that the result of promoting grain growth is insignificant.